Questions on microwave ovens being used for magnetron and power supply

[richjc]

I'm working for a small company and they want to do a cheap experiment/feasibility study that requires some microwave sources.

Currently they are thinking of getting some cheap microwave ovens from the store and taking out the magnetrons and power supply for the microwave source. I have some questions regarding this approach that I hope someone can help me with.

1. Does anyone foresee any problems in adapting the power supply in the microwave oven to power the magnetron continuously?

2. Does anyone foresee any problems with the magnetron being powered consciously? Heat issues etc with consciously being powered.

3. How do the different power settings on a microwave adjust the power delivered to the food? Is just how long it leaves the magnetron on?

4. I seem to remember you are supposed to "bake" in magnetrons... if you get a new microwave oven, do we have to perform the bake in process also?

5. The magnetron will be coupled to a wave guide using a 1/4 wavelength antenna and measuring the length of the antenna will allow us to determine the exact frequency of the microwaves emitted?

6. A conjecture on the peak E-Field strength inside the waveguide before it reaches the oven cavity for an 800Watt or 1000watt microwave oven


Thanks in advance for any responses.

 

[logbook]

>2. Does anyone foresee any problems with the magnetron being powered consciously? Heat issues etc with consciously being powered.

I assume you mean "continuously" not "consciously".
They do run continuously when running at the 100% power setting don't they?

>3. How do the different power settings on a microwave adjust the power delivered to the food? Is just how long it leaves the magnetron on?
I have seen one doing a duty cycle adjustment based on a cycle time of at least 20 seconds, such that 50% is 10 seconds on and 10 seconds off, but this can't be generalised to all microwave ovens.

>5. The magnetron will be coupled to a wave guide using a 1/4 wavelength antenna and measuring the length of the antenna will allow us to determine the exact frequency of the microwaves emitted?
The specification will be more accurate than this measurement.

>6. A conjecture on the peak E-Field strength inside the waveguide before it reaches the oven cavity for an 800Watt or 1000watt microwave oven.
This should be a straightforward calculation.

 

[richjc]

Hi,

Thanks for the quick response.

Yeah I meant continuously, guess that’s what happens when you just get click happy with the spell checker and not read.

Yeah I was being lazy with question 6, thought I would just add it as a question as I was posting some anyway.

When you said the specification would be more accurate, is that going to be listed on the microwave oven its self or on the magnetron or something I would have to look up after I know the manufacturer etc?

Thanks

 

[logbook]

Power= (E field max)*(E field max) * b * a / (4 * Z)

a= wide dimension of guide
b= narrow dimension of guide

Z= 377 / sqrt( 1 - (lambda/2*a)*(lambda/2*a) )

 

[Higgler]

Do you plan on using multiple ovens at the same time for heating or drying?


kch

 

[richjc]

Thanks logbook, info much appreciated... I am a little curious on the formulas though... Do you know where the 377 comes from? Is the formulas setup for SI units? If I wanted to locate these formulas in a books index or online so I can see it being derived what would I look for?

Higgler the study is not connected with heating or drying, we just do not want to cause heat damage to the magnetrons.

Thanks

 

[logbook]

377 ohms is the characteristic impedance of free space.
Units are SI.

Ref: Fields and Waves in Communication Engineering, 3rd ed, Ramo, Whinnery and van Duzer. Section 8.8, page 425 equation 14.

The parenthesis on the Z is not quite right.
Should be:
Z= 377 / sqrt( 1 - (lambda/(2*a))*(lambda/(2*a)) )

Look for power flow in TE10 mode.

Of about ten microwave books I checked, only two had a formula.

 

[richjc]

Thanks logbook.

 

[dontthinktwice]

Logbook is right: 377 ohms is the impedance of free space. It is actually the square root of the permitivity of free space over the permability of free space, or:

Z0 = sqrt(mu0 / eps0) = 377 ohms (approximately)

The permitivity and permbility are more fundamental concepts than the impedance.

 

[pillman]

is there a way to turn a solid ( eg. a cracker) in to a powder with out actualy crushing it

 

[BrianG]

Pillman - do not hijack someone else's post! Start your own topic in the proper forum if you want to get an answer.